Elastomer-reinforced polymers of vinyl aromatic compounds, such as styrene, alpha-methylstyrene and ring-substituted styrene, have found commercial use. For example, elastomer-reinforced styrene polymers having discrete particles of cross-linked elastomer dispersed through the styrene polymer matrix may be useful for food packaging, office supplies, point-of-purchase signs and displays, housewares and consumer goods, building insulation, and cosmetics packaging. The incorporation of an elastomer into the styrene matrix may result in improvements in a range of physical and mechanical properties (e.g., impact strength) and collectively these polymers are termed high impact polystyrenes.
The utility of a particular high impact polystyrene may depend on the polymer having some combination of mechanical, thermal, and/or physical properties that render the material suitable for a particular application. These properties are related in part to the extent of incorporation of the elastomeric material into the polymer matrix. Many factors during polymerization can affect the properties of polymer. Once such factor is the degree of crosslinking in the rubber phase, which may result in decreased impact resistance, and environmental stress cracking resistance, which may be reflected by a lower swell index.
Some crosslinking may be desired for low to medium viscosity rubber to stabilize the rubber particle morphology through the devolatilization process. However, excess crosslinking may alter the elasticity of the elastomer phase and be detrimental to the final properties of high impact polystyrenes, such as impact strength and environmental stress cracking resistance.
Crosslinking control agents may be introduced to control the crosslinking in of high impact polystyrenes. However, some crosslinking control agents may cause discoloration in the resultant high impact polystyrene.